Multiplexed optical signals, such as high-speed wavelength division multiplexing (WDM) signals at 100 Gbps or the like, have spectra that vary depending upon the wavelength spacing between multiplexed channels. For example, when a channel to be multiplexed is added to a WDM signal, the added channel and an adjacent channel in operation, which is adjacent to the added channel, interfere with each other, resulting in a reduction in the signal quality. A technique for reducing interference by increasing the wavelength spacing between the channels in the WDM signal is conceivable in order to enhance the signal quality. However, if the wavelength spacing is excessively increased, the number of channels in which wavelengths can be multiplexed decreases. Consequently, the transmission capacity decreases, and the wavelength utilization efficiency also declines.
Accordingly, there is a technique for adjusting the signal quality and the wavelength utilization efficiency in a WDM signal. One example of such a technique is a method for adjusting the channel spacing for each wavelength while monitoring the signal quality corresponding to the wavelength spacing between the channels in a WDM signal.
Examples of related techniques are disclosed in Japanese Laid-open Patent Publication No. 2012-23607, Japanese Laid-open Patent Publication No. 2002-44021, Japanese Laid-open Patent Publication No. 2011-228819, and Japanese Laid-open Patent Publication No. 2010-098544.